Multiplex communication systems



Relative Aflenuation-Decibels MULTIPLEX COMMUNICATION SYSTEMS FiledApril 22, 1959 3 Sheets-Sheei 3 will D 9 8 fc I .2

9 m l O I l I l l I l I l I I I 5 I l I I I -IO -6 -2 fc 2 6 l0 8 4 O 48 Deviation From Cenier Frequency Deviation From Center FrequencyKilocycles Per Second Kilocycles Per Second Fig.6

Fig.7

Sindhi??? Patented. Dec. id, 3952 lice lav-i The present inventionrelates generally to multiplex radio communication systems and moreparticularly to systems for simultaneous transmission of two signals asseparate modulations of a single carrier and for reception of suchsignals at a remote location without undesirable interferencetherebetween.

The present invention finds one particularly advantageous application inradio transmission of stereophonic sound s gnals. In ordinary radiosystems, sound from a single microphone is transmitted over a singleradio channel having a frequency bandwidth of about 9 to 10 kilocycles.In such systems, audio perspective is entirely lost since the amplitudediiference, time delay and phase displacement between the soundsreceived by the two ears of the listener bears no relationship to whathappens at the microphone which feeds the transmitter. Stereophonictransmisison and reception has heretofore been demonstrated using twomicrophones, set up at 10- cations on each side of a stage on which anorchestra, for example, may be situated. Each microphone is connected bya separate radio channel to one of two loudspeakers placed similarly aswere the microphones, but in a listening chamber where the receivingapparatus is situated. By such previously demonstrated arrangements, anauditory effect may be obtained which is substantially the same asthough the orchestra or other source of sound were actually located infront of the listener rather than the sound being reproduced by theloudspeakers.

In applying sterophonic sound concepts to radio communication, one veryserious obstacle is the need for two separate channels for thetransmission of a single entertainment program. That obstacle hasheretofore prohiblted commercial stereo transmission in the AM broadcastband. To receive commercial acceptance, any broadcast band stereophonictransmission system must conform to the requirement that all signalsoutside a single frequency band of about 9 kilocycles bandwidth should,by international agreement, be attenuated. so that broadcasttransmitters in adjacent channels are not disturbed. Desirable solutionsto the problem would permit transmission of both audio channels over thesame carrier frequency, thereby using but one radio channel and reducingto a minimum the additional investment required at the transmitter aswell as the additional investment required by prospective listeners.

One. proposed system using a single channel is described in detail inElectronics Magazine, issue of February 1941, at pages 34 to 36. Thatsystem suggests tranmission of the audio signal from a first microphoneas amplitude modulation and that from a second microphone as phasemodulation of the same carrier. Such a system has the disadvantage thata conventional receiver will reproduce the signal from the firstmicrophone only. Thus, a conventional AM receiver would produce soundsign ls corresponding to those heard at one end of a stage on which theorchestra is located. A primary requisite of a genuinely compatiblesystem is that a conventional receiver should produce balancedmonophonic sounds substantially corresponding to the sound effects whichwould be heard by a listener seated near the center of the studio inwhich the orchestra is located. Another disadvantage of theabove-mentioned amplitude and phase modulation system is that phasemodulation necessarily requires a greater bandwidth in order to producethe same signal to noise figure. That is, if phase modulation is used,the frequency deviation of the carrier signal in creases as thefrequency of the audio modulation. increases. Accordingly, if theavailable bandwidth were fully utilized at low modulation frequencies,then it would be exceeded at the higher frequency audio modulations. Ifthe available bandwidth were fully utilized at the maximum audiomodulation frequencies, then it is not exploited at the lowestmodulation frequencies and the signal to noise ratio would suffer. Thusphase modulation systems are practically incompatible for use in theamplitude modulation broadcast band.

In another prior art stereophonic transmission system, signals A and Bfrom spaced microphones are trans mitted by adding the signals andtransmitting the resulting sum signal A-l-E as frequency modulation of acarrier and simultaneously subtracting the signals and transmitting thedifference signal A-B as frequency modulation of a supersonic subcarrierin the same channel. At the receiver, the two signals after processingby dififerent circuits, are respectively demodulated and then (1) addedto produce the first audio signal A, and (2) algebraically subtracted toproduce the second audio signal. The sum and difference signal conceptused by that system has the advantage that a single receiver tuned tothe A+B channel will provide normal monophonic sound reproduction.However, such a system has the important disadvantage of requiring abandwidth of several tens of kilocycles to simultaneously accommodatethe main frequency modulated carrier and also the supersonic subcarrier.it is not suitable for use in the AM broadcast band where bandwidth isrestricted by international agree ment to approxl'rnately l0 kilocycles.Accordingly, such a system would be usable commercially only in thepresent PM broadcast bands or in other high frequency bands and wouldnot provide compatible monophonic reception by means of conventional AMbroadcast receivers. In addition, the last-mentioned system requirescircuitry of special design and considerable expense for stereophonicreception of the subcarrier signal.

Accordingly, a primary object of the present invention is to provide atransmission system affording compatible reception of monophonic soundfor listeners having conventional AM broadcast band receivers andsimultaneously providing stereophonic reception for listeners havingreceivers in accordance with the invention.

it is a different primary object of the present invention to provide asystem for transmission of a plurality of intelligence bearing signals,wherein one of the signals is utilized to amplitude modulate a carrierand another signal is utilized to angle modulate the same carrier.

it is another object of the present invention to provide a multiplexcommunication system for more efi'iciently utilizing radio frequencychannels wherein first and second signals respectively modulate a singlecarrier in amplitude and frequency respectively, and whereinpredistortion of one of the modulations is provided to substantiallycompensate for distortion which would otherwise occur in the process ofdetection of said one modulation by means of a conventional receiver.

It is a further object of the invention to provide a multiplex radiocommunication system in which first and second signals are transmittedsimultaneously as amplitude and frequency modulation respectively of asingle carrier wave and in which signal components are provided toprecorrect the transmitted modulation for distortion expected to occurin the receiver.

it is an additional object of the present invention to provide a systemof communication wherein first and second separate signals aretransmitted simultaneously by frequency modulation and by amplitudemodulation respacers spectively of the same carrier and wherein theamplitude modulation is modified before transmission in accordance witha distortion compensating signal which corresponds inversely to theamplitude distortion expected at the receiver whereby conventionalamplitude modulation receivers can reproduce said first signal withoutinterference from the frequency modulation.

It is a still further object of the present invention to provide, interalia, a stereophonic transmitter apparatus for use in the conventionalAM broadcast band which apparatus may be constructed by addition ofinexpensive auxiliary components to an existing broadcast band amplitudemodulation transmitter.

Other general objects of the present invention are to provide animproved stereophonic sound radio transmission system, to apply theforegoing objects to such stereophonic systems, and to provide forcompatible reception of monophonic sound by listeners who are notequipped with special stereophonic receiving equipment.

Briefly described, the present invention transmits a plurality ofinformation bearing signals on a single communication channel by addingthe signals and transmitting the summation as amplitude modulation andsubtracting the signals and transmitting the resulting diflerence signalas frequency modulation of the same carrier. In specific application tostereophonic sound transmission, instead of transmitting the output ofone microphone over one channel and the output of the other microphoneover the other channel, the sum of the outputs of the two microphones istransmitted as amplitude modulation and the difference of the outputs istransmitted as frequency modulation of the same carrier. In a preferredembodiment the difference signal is filtered prior to application to thetransmitter so that only audio signals substantially within the range of300 cycles to 3000 cycles are transmitted. Such transmission enables aconventional AM receiver tuned to the transmitter carrier, to reproducea sum signal A-l-B which contains equal components of the signal fromeach microphone and therefore represents balanced monophonic sound.

For stereophonic reception, at special receiver is provided whichrequires only an amplitude limiter and a frequency detector in additionto the conventional circuits of a standard amplitude modulationbroadcast band receiver. The A-i-B signal from the conventionalamplitude detector and the AB signal from the frequency detector arematrixed by known sum and difference producing circuits to reproduce thesound signals A and B separately. In the system of the present inventionas thus far described, one particular problem of note arises: If it beconsidered for the moment that there is no amplitude modulation and onlyan FM modulation, such as would occur when A and B are equal and out ofphase, it is seen that the frequency selective circuits of aconventional receiver will modulate the amplitude of the received PMsignal. More precisely, as the frequency modulated car rier sweepsacross the receiver passband from one extreme frequency deviation to theother extreme frequency deviation, it will encounter portions of thereceiver passband having varying gains. Accordingly, the output of thefrequency selective network of the receiver will be amplitude modulatedas a function of the absolute frequency deviation of the carrier signalfrom its center frequency. This spurious amplitude modulation appears inthe signal applied to the amplitude detector as frequency-amplitudedistortion or cross-talk into the A-l-B channel. To overcome theforegoing cross-talk problem, the present invention providesprecorrector means at the transmitter to insert an amplitude modulationcomponent of sufficient amplitude and character to substantiallycounteract that which is expected to occur in the frequency selectivenetwork of an average AM receiver.

The foregoing and other objects and features of the present inventionwill be apparent from the following description taken with theaccompanying drawing, throughout which like reference charactersindicate like parts, which drawing forms a part of this application, andin which:

FIG. 1 is a functional block diagram of a radio transmitter arranged inaccordance with the present invention;

2 is a functional block diagram of a radio receiver arranged to receiveand demodulate the signals transmitted by the transmitter illustrated inFIG. 1;

PEG. 3 is a functional block diagram of a modification of thetransmitter illustrated in MG. 1;

MG. 4 is a functional block diagram of a further transmitting apparatusarranged in accordnace with the present invention;

PEG. 5 is a schematic diagram of an additional type of circuit forperforming the same functions as certain elements of the transmitters ofFIGS. 3 and 4; and,

H68. 6 and 7 are graphs illustrating certain typical frequency responsecharacteristics useful in explaining the various features of the presentinvention.

Referring now more specifically to FIG. 1 of the drawings, the referencenumeral denotes a source of modula tion signal, which may be denominatedintelligence sig nal Y. The modulation signal Y provided by source ltlis applied to an amplitude modulator is which may comprise, for example,circuits of conventional character commonly used in transmittingstations for applying amplitude modulation to carrier waves. Referencenumeral 12 denotes a second source of modulation signal, which may bedenominated intelligence signal X. The signal X from source 12 isapplied to a frequency modulation system 14, 16 of conventionalcharacter which may in clude the frequency oscillator 14 and a balancedmodu lator circuit 16 which circuits are known per se. Alternatively,the frequency modulation system may comprise an oscillator associatedwith a reactance tube for varying the frequency of the oscillator inaccordance with the signal from source 12. The frequency modulatedcarrier wave from frequency modulator 16 is applied to amplitudemodulator l8 and is amplitude modulated by the signal from signal sourceill. The resultin amplitude and he quency modulated signal is applied toa notch filter network 2% having a frequency response characteristicgenerally as shown by curve 21. As shown by curve 21, network 24;operates on a carrier wave to attenuate the carrier frequency to agreater extent when it is near the center frequency f of the oscillator14 than when it is deviated to a maximum deviation A That variable attenuation characteristic of the network 2 3 provides am-' plitudemodulation of the carrier wave in accordance with a positive function ofthe absolute deviation of the instantaneous carrier frequency from thecenter frequency f The resulting carrier wave, amplitude modulated inaccordance with the signal Y plus the correction signal, and frequencymodulated by the signal X is applied from the output of network it) tothe input of a conventional linear power amplifier 22 and thence to anantenna 24 which radiates the signal in conventional mannet. Theradiated carrier signal thus includes frequency modulation componentscorresponding to the signal from source 12, amplitude modulationcorresponding to the signal Y from source lltl, and a further amplitudemodulation predistortion component supplied by the action of etwork 2d.The radiated carrier may have a nominal or center frequency i of, forexample, 1000 kilocycles. The carrier frequency deviation M ispreferably limited to plus and minus 3 kilocycles. The amplitudemodulation of the carrier signal is preferably limited to approximatelyin PEG. 6 there is shown a plurality of curves illustrating thefrequency response characteristics of certain conventional AM receivingsets. Curve 31 represents the frequency response characteristic orbandpass characten istic of an average amplitude modulation receivingset having a handwith of approximately 8 kilocycles between the 6decibel attenuation points 33 and 35. When a com posite signal of thetype produced by the circuit system of FIG. 1 is applied to a receiverhaving the bandpass characteristic as shown by curve 31, the frequencydeviations which are a consequence of the stereophonic frequencymodulation cause the carrier to sweep back and forth across the curve 31at least part way between the points 33 and 35. Accordingly, the gain ofthe receiver will be varied as a function of the frequency deviation andspurious amplitude modulation would be introduced and detected by theconventional AM detector circuit. The spurious amplitude modulation ordistortion of the received signal by the receiver passbandcharacteristic may result in cross-talk of the frequency modulatingsignal X into the amplitude modulating signal Y as the signal Y isreproduced by the amplitude modulation receiver. Under normalconditions, the cross-talk distortion into the amplitude modulationsignal Y will be principally second harmonics of the frequencymodulating signal X. The notch filter network precorrects for theaforesaid distortion expected in the receiver by providing an amplitudemodulation predistortion component substantially corresponding inverselyto the distortion expected in an average AM receiver. Such predistortionis ac complished by providing the network 20 with a frequency responsecharacteristic substantially as shown by curve 21 in FIG. 1 or by curve41 in FIG. 7. From inspection of FIGS. 6 and 7 it may be observed thatresponse curve ill of the network 2% is substantially the reciprocal ofthe receiver frequency response curve 31 as shown in FIG. 6. Specificcircuit arrangements for providing the network 2% with a frequencyresponse characteristic such as that shown by curve 41 will be describedin greater detail hereinafter in connection with the embodiments of thepresent invention as shown in FIGS. 4 and 5.

Referring now more specifically to FIG. 2 of the accompanying drawing,the reference numeral 26 denotes a conventional receiving antenna forreceiving signals transmitted by the apparatus of FIG. 1. Signalsintercepted by antenna 26 are applied to a conventional heterodyneconverter and intermediate frequency amplifier system denoted by block28. The received radio frequency signals are converted byconverter-amplifier 28 to an intermediate frequency signal having thesame modulations as the original carrier signal. The intermediatefrequency carrier signal is amplified by and applied from block 28 to anamplitude detector 39 and also to an amplitude limiter 34 both of whichare connected to the output circuit of the intermediate frequencyamplifier.

A first signal channel of the receiver of FIG. 2 includes conventionalamplitude detector 30 and audio amplifier 32 for detecting the amplitudemodulation signal Y and supplying that signal to a signal combiningmatrix 46. A second signal channel, comprising a limiter circuit 34, afrequency modulation detector 36 and an audio amplifier 38 all connectedin cascade to the output of the intermediate frequency amplifier,operates to detect the frequency modulation signal X and apply thesignal X to a second input of the matrix at One signal combiningmatrixing network of the type suitable for the block 4d of FIG. 2 isdisclosed and described in detail in an article entitled SinglePush-Pole for Stereo Channels, published in Radio and Television News,issue of January 1959 at pages 48 and 49. It will be apparent to thoseskilled in the art that addition and subtraction of signals by means oftransformer arrangements as shown in the above-mentioned article is notessential to the present invention. Other arrangements, known per se,utilizing resistance networks or phase inverters and additive amplifiercircuits may also be used in the system of the present invention. Thesignal combining network 4d has first and second output circuitsconnected respectively to first and second sound reproducing devices 42and 44. Sound reproducing devices 4 2 and id are shown as comprising apair of loudspeakers preferably spaced apart in a listening space suchas a room of the listeners home. Such spaced loudspeakers will, ofcourse, be used only when the transmitted signals X and Y arerespectively the stereophonic and monophonic components of a transmittedstereophonic program material, In accordance with other aspects of thepresent invention wherein the signals X and Y may be entirely unrelatedinformation signals, the reproducers 42 and 44 may comprise unassociatedsound reproducing devices, or information signal recorders of variousknown types.

The frequency modulation detector circuit as may comprise any of variouswell known frequency discriminator circuits such as, for example, agated beam detector. Similarly, the limiter 34 and the audio amplifier38 will be recognized as components similar to those of a conventionalFM receiver which serve to demodulate the PM carrier to produce an audiofrequency signal which is amplified and fed to the combining circuit id.The only essential criteria for the second signal channel of thereceiver is that the frequency modulation detector 36 should be arrangedto demodulate carrier waves of frequencies in the conventional 1F band,normally about 4-56 kc.

Referring now to FIG. 3 of the drawing, microphones A and B are the twospaced microphones of the stereophonic system which microphones may bepositioned in spaced relation on the stage on which an orchestra or thelike is located. The outputs of microphones A and B are amplified byaudio preamplifiers 46a and 46b respectively and the amplified audiosignals are applied to first and second input circuits of a sum anddifference matrix 48. Network 48 preferably comprises one of variousknown arrangements using resistance networks or phase inverters andamplifier circuits for producing a stereophonic difference signal A-B atoutput terminal 45, and a monophonic sum signal A+B at output terminald7. The sum signal A-l-B corresponds to the monophonic sound informationwhich would be heard by a listener seated near the center of theauditorium in which the orchestra is located, and may be considered ascorresponding to the first information signal Y from the source it? ofFIG. 1. Similarly, the stereophonic difference signal A-B at terminal 45may be considered as corresponding to the sec- 0nd information signal Xderived from the source 12 of FIG. 1.

The stereophonic difference signal /1B is applied from terminal 45 tothe input of a frequency modulated oscillator 49 which preferablycomprises aconventional oscillator controlled by a reactance tubecircuit. Frequency modulated oscillator 49 is preferably designed toprovide a deviation of approximately 3 kilo-cycles maximum from thecarrier center frequency in response to stereophonic difference signalsfrom terminal The frequency modulated carrier is supplied to a notchfilter network 2% corresponding to that of FIG. 1. Summation signalA-l-B, from terminal 47, is supplied by way of a phase Corrector 54 to aconventional amplitude modulator It will be understood that thearrangement of terminals 45 and 47 could be reversed with the summationsignal A+B being used to modulate the carrier frequency; however, thearrangement as shown in FIG. 3 is preferred when the system of thepresent invention is used for stereophonic transmission of sound inorder that amplitude modulation receivers of conventional. type willreceive the monophonic signal A-l-B rather than the difference signalA-B. If the terminals 5 and 47 were so reversed the system of FIG. 3would not be compatible with conventional amplitude modulationbroadcasting.

The output signal from notch filter network 2t} comprises a frequencymodulated carrier signal having an amplitude modulation corresponding tothe absolute deviation of the carrier signal instantaneous frequencyfrom its center frequency f Such amplitude modulation is produced by thefrequency response characteristic of the network 20 as describedheretofore with reference to MG. 1. The carrier signal channel of thetransmitter of PEG. 3 further comprises a class C amplifier 50, a firstamplitude modulator 18, and a second amplitude modulator 18' connectedin cascade between the output of network 2% and a transmitting antennaIndividually, these components will be recognized as known components ofa conventional amplitude modulation transmitter which here serve topower amplify the frequency modulated carrier signal and to applyamplitude modulation intelligence thereto for radiation by the antenna24.

High power transmitters of the type exemplified by amplifier 5t? and themodulators l8 and 18 do not, as a general rule, provide a fiat frequencyresponse characteristic. Accordingly, it is not desirable to pass theamplitude modulation predistortion component provided by network 2t]through the transmitter circuits 5 9 and 18. The system of FIG. 3overcomes this problem by the provision of the amplitude modulationdetector 52, in the auxiliary signal path extending from network 2t! toa second input of the amplitude modulator 18. Since amplifier operatesclass C, it does not transmit the amplitude modulation predistortionsignal but rather translates only a frequency modulated carrier with thefrequency modulation corresponding to the A-B signal. The carrier signalfrom network 2% which is amplitude modulated with the desiredpredistortion signal is translated by the auxiliary signal path showndiagrammatically as conductor 51 to the input of a conventionalamplitude modulation detector 52. The output signal from detector 52corresponds to the amplitude modulation envelope of the carrier signalfrom network 2% and is an audio frequency signal corresponding to thedesired predistortion correction. Thus, the output from detector 52 asapplied to the second input 53 of amplitude modulator 18 constitutes apredistortion control voltage which varies as a function of the absolutefrequency deviation and in the usual instance is rich in secondharmonics of the stereophonic difference signal AB. Since the distortioncontrol voltage is a positive function of absolute deviation, it willhave a maximum value when the difference signal A-B is maximum and willfall to zero when the difference signal is zero.

An outstanding advantage of the compensation system of the presentinvention when used in a stereophonic sound transmission system of thetype shown in PEG. 3 utilizing the A+B and the AB concept is that thepredistortion control voltage will always be zero when the amplitudemodulation due to the sum signal A+B is maximum. That characteristicenables high level amplitude modulation of the carrier signal by meansof modulator 18' so that the monophonic sum signal A-l-B may betransmitted with maximum amplitude modulation. Accordingly, theamplitude modulation sidebands as radiated by antenna 24 will have powerlevels approaching 50% of the total radiated power, as is the usual casein ordinary AM broadcasting, and the transmitter of FIG. 3 will havesubstantially the same broadcast range as a conventional monophonictransmitter.

The foregoing advantage will be better understood by a detailedconsideration of the characters of the monophonic sum signal Y=A+B, andthe stereophonic difference signal X =AB. Consider first the case inwhich the sound at the two microphones is balanced, i.e. equal and inphase. Now, assume that the Y signal amplitude at terminal 4'7 necessaryto produce, for example, 95% amplitude modulation is equal to 1. Sincethe sound, in this case, is balanced it follows that signal component Ais equal to B is equal to 0.5, and the stereophonic difference signalX=AB=0. Thus, when the sound is balanced, the difference signal is Zeroand the carrier may be safely modulated at levels approaching 190%modulation without danger of overmodulation by the predistortion controlsignal.

Considering next the case where the sound signal amplitude at microphoneA is maximum and the sound signal amplitude at microphone B is alsomaximum but 180 out of phase with that at microphone A. The difference 8signal X has a maximum value but the sum signal Y has a minimum value,namely zero. Since the difference signal is a maximum, the predistortioncontrol signal is also a maximum but no danger of overmodulation existsbecause the sum signal Y is Zero.

Reference is now made more specifically to FIG. 4 of the accompanyingdrawing. EH}. 4 illustrates a transmitter, which is of the generalcharacter of that illustrated in FIG. 3 in that a source firstmodulation signal X==AB is utilized to modulate the frequency of thecarrier supplied by frequency modulated oscillator 49, and in that asecond modulation signal Y=A+B is applied through a phase correctornetwork 54 for amplitude modulating the carrier wave by means of anamplitude modulator The system of PEG. 4 differs in that the output thefrequency modulated oscillator 49 is applied direet y to a class C poweramplifier till and then to the a ,ntude modulator 13 rather than beingapplied tirough the notch filter network 2% as in FIG. 3. The system ofFIG. 4 is preferable to those of FIGS. 1 and 3 in that the expensivehigh power level components of the system of FIG. 4 may be identical tothose of a conventional amplitude modulation transmitting station. InFIG. 4 the dotted block designates the high power level section or" thetransmitter for applying power to the radiating antenna 24-. Block 57incorporates the usual class C power amplifier 5t? and the usualamplitude modulator stage lid. The frequency modulated carrier signalfrom oscillator d9 is applied by way of a terminal r33 rectly to theinput circuit of class C amplifier 5t and the frequency modulatorcarrier signal, Without amplitude modulation is applied from amplifier5% to the carrier si, ial input circuit of the amplitude modulator K8.

in addition, the system of PEG. 4 incorporates a bandpass filter network56 connected between the source of stereophonic difference signal X==ABand the input to the freque cy modulating oscillator 49. Listening testsusing both earphones and speakers have indicated that there is littlestereophonic information detectable by the listener in audio signalsbelow about 360 cycles per second. Consequently, the stereophonicdifference signal AB will be of extremely small amplitude when thefrequencies of the signal are below about 300 cycles per second.Further, it has been found, by appropriate tests that excellent stereoeffect is produced when audio frequencies above 3000 cycles per secondare present in the monophonic channel only. Thus, very littleimprovement in stereo efiect would he achieved by transmitting thesignals below approximately 300 cycles per second or the signals aboveapproximat ly 30% cycles per sec- 0nd. in a preferred embodiment of thepresent invention, the bandpass filter 56 may comprise a conventionalresistance-capacitance filter network having a bandpass characteristicextending from 300 cycles to 36% cycles between the 3 decibelattenuation points. The filter cutoil rate, outside the desiredbandpass, preferably should be approximately 6 decibels per octave.

In the system of FIG. 4 the bandpass filter 56, the modulated oscillator4-9, amplifier 5d and modulator l3 comprise the frequency modulationchannel of the transmitter. The predistortion control voltage generatingsystem of 4 comprises a notch filter network 5% connected in cascadewith a conventional amplitude modulation detector and a phase correctingdelay line as to a first input of an adder circuit 66. The particularnotch filter network used the embodiment of FIG. 4- comprises adouble-tuned, over-coupled radio frequency transformer having a primarywinding which is shunted by a tuning capacitor 59 and having a secondarywinding wlt ch is shunted by a tuning capacitor 62. One end of primarywinding 6% is coupled to output terminal 63 of the frequency modulatedoscillator The other of winding is connected to ground or to a point ofreference potentia. One end of secondary winding 61 is connected to thesame point of reference potential and the other end is coupled to theinput circuit of the detector 52. The notch filter 58 ShO.lld. have abandpass characteristic substantially as shown by the curve 67 at theleft of the network 58 in FIG. 4. As shown by curve 67, thedouble-tuned, over-coupled transformer network has a reentrant notch inits center portion, so that the fre quency modulated carrier signal fromterminal 63 will e relatively more attenuated when its instantaneousfrequency is near the center frequency f than when near the points ofmaximum frequency deviation A7. Preferably the curve of network 58should be substantially identical to the curve ill of FIG. 7 between the4 decibel attenuation points. Filter 58 comprising the over-coupleddouble-tuned transformer will be recognized as a structure known per seto those skilled in the art. The design of such bandpass filter networksis described in full in Terman, Radio Engineers Handbook, section 3,paragraph 9, first edition, 194-3. Thus, the bandpass frequency responsecharacteristics of the network 53 is substantially inverse to that of anaverage monophonic AM receiver as exemplified by the curve 31 in FlG. 6.Accordingly, the network :38 has substantially the same function as thenotch filter network 2t? of FIG. 1 and produces at its output afrequency modulated carrier wave which is amplitude modulated as adirect function of the frequency deviation. That amplitude modulation isdetected by the conventional detector 52 and provides an audio frequencydistortion control voltage at terminal 65 which control voltage issubstantially the same as that described in detail as being applied toterminal 53 in FIG. 3. The predistortion control signal from terminal 65is applied, through a phase correcting delay line 6 5, to a first inputof an adder circuit 66. Simultaneously, the monophonic sum signal Y=A+Bis applied through phase corrector 54 to a second input of the addercircuit The phase correctors 54 and 64 provide appropriate phase shiftor delay of the respective signals so that the A-l-B amplitudemodulation of the carrier will have the proper phase relationship to theA-B frequency modulation thereof, and so that the predistortion controlsignal will provide the maximum predistortion modulating e -.ct insynclironism with the maximum frequency deviations of the frequencymodulated carrier. Thus, any phase difi'erence arising from thedifierence in the channels traversed by the monophonic signal Y and thestereophonic signal X is taken care of in the transmitter, and no phasecorrection is required in the receiver of HG. 2. Adder circuit combinesthe monophonic signal Y with istortion control signal to provide apredlstorted amplitude modulating signal which is applied from theoutput of adder as to a second input of amplitude modulator ES tomodulate the amplitude of the carrier wave. Accordingly, the carrierwave at the output of modulator l3, and as radiated from '24, includespredistorted amplitude modulation with the predistortion beingsufficient so that the distortion occurring in an ordinary AM receivingset is counteracted by the predistortion. Accordin ly, persons havingordinary amplitude modulation receiving sets may receive the monophonicsignal Y independently of and without interference from the stereophonicsignal X which is simultaneously transmitted on the same carrier byfrequency modulation.

Although notch filter network 53, as shown in FIG. 4, will provide anentirely satisfactory predistortion control signal when properlydesigned, the invention is not dependent upon the use of the doubletunedover-coupled transforme arrangement. The same advantages of the p esentinvention and the same predistortion control signal may be obtained by acircuit such as that shown in FIG. 5. In certain application, thecircuit of FIG. may be preferable to that of P16. 4 in that the circuitof FIG, 4 requires transformer coils of relatively high Q in order togive the proper bandpass frequency response ill characteristic. Also,difliculty may be encountered in providing adjustability of thefrequency response characteristic of the filter 58, The circuit of HG. 5provides a notch filter network of adjustable Q so that the width of thebandpass notch may be adjusted and further provides adjustment of thepredistortion control signal amplitude so that the effective depth ofthe bandpass notch is controllable.

As shown in MG. 5 the notch filter network comprises radio frequencyamplifier tubes 6% and 78 connected in cascade with frequency modulatedcarrier signal from the frequency modulated oscillator being applied byway of terminal 63 and through resistor ill shunted by capacitor to thegrid of the tube 68. The cathode of the conventional pentode 63 isconnected to ground or a point of reference potential by cathode biasresistor '72 shunted by capacitor 74. The anode of tube 68 is connectedto a source of energizing potential 8+ through a plate load resistor andis further connected to the grid electrode of conventional pentode 78through a coupling capacitor 76. The screen grids of pentode es and 73may of course be provided with the proper operating potential by meansof conventional circuitry which has been omitted for simplicity.

To provide the desired frequency response charactersubstantially asshown by curve ll in FIG. 7, the amplifier 58 is provided with adegenerative feedback circuit including resistors fill and 32 connectedserially between capacitor 76 and the grid of tube 68, with resistor 82being shunted by capacitor 83, and with a single tuned circuit,comprising radio frequency inductor 84 shunted by a variable capacitor85, connected from the junction of resistors bit and 32 to ground. Aso-called Q-multiplier circuit comprising a conventional pentodedischarge device 86 is coupled across a portion of the inductor by meansof coupling capacitor 37 and connection of the cathode to anintermediate terminal of inductor Tube as and its associated circuitryoperate ve resistance across the tuned circuit 8 and 35, therebyincreasing the effective Q of the coil 84 and enabling a notch of anydesired sharpness to be formed in the passband characteristic of thecascade amplifiers 63 and '73. The cathode of tube 36 is returned toground through a portion of inductor 84. The anode is supplied withenergizing potential from a conventional source of B+through a loadinductor 88. The grid of tube 86 is bypassed to ground by capacitor 89and is connected through a current limiting resistor 99 to the variabletap of a potentiometer 91 the ends of which are connected respectivelyto ground and to a source of negative biasing potential. Ad'ustment ofpotentiometer 91 changes the bias on the grid of tube as, therebyvarying the gain of tube as and effectively varying the value ofnegative resistance supplied across the coil 34- by the Q-multipliercircuit.

Resistor Si; and tuned tank constitute an RF voltage divider extendingfrom the output circuit of tube 68 to ground. intermediate terminal 79of the voltage divider. is connected through resistor SE) to the tube68. Thus, the amount of degenerative feedba applied through resistor tothe grid of tube will vary as the function of the effective impedancefrom terminal 79 to ground. The tank circuit 84, 85 is tuned to thecenter frequency of the carrier wave. Accordingly, when the carrier wavetranslated by tube is near the carrier center frequency f the tankcircuit dd, will exhibit a maximum impedance and a maximum amount ofnegative feedback voltage will be applied through resistor 82 to thegrid of the tube 68. Accordingly, the n of amplifier 63 will be at aminimum when the carrier wave applied to terminal 63 is at the centerfrequency f When the carrier wave instantaneous frequency deviates fromthe center frequency f in response to increase .plitude of the frequencymodulated carrier signal.

' plitu de modulation receiver.

aoeaeva f. a in the absolute value of the stereophonic difference signalAB, the tank circuit will exhibit a lesser impedance to the off centercarrier wave frequency, and accordingly a lesser amount of negativefeedback voltage v. be applied from terminal '79 through resistor 82 toinput of tube Thus, the effective gain of the ra frequency amplifiercircuit comprising tubes i655 and varies substantially as shown by curve41 in FIG. 7, and corresponds inversely to the amplitude frequencyresponse characteristic of an average receiver as reprc ented by thecurve 31 in FIG. 6. Such inverse correspondence should hold true atleast between the 4 decibel points of curve I The portions of curve 41corresponding to excessive ation from the center frequency i are showndotted to indicate that the frequency response characteristic suchregions is not of particular importance.

From the foregoing it will be apparent it at the frequency modulatedcarrier signal Output from tube '78, as developed across anode loadresistor 92, has an amplitude envelope which varies as an absolutefunction of the frequency deviation of the carrier signal from thecenter frequency f The anode of tube 73 is coupled through capacitor 93to the input circuit of a conventional amplitude modulation detector 52comprising rectifier device 95 and a pi filter network includingresistor 96 and capaci tors 97 and 9&3. Detector circuit 52. demodulatesthe carrier signal to produce across output capacitor 98 a distortioncontrol voltage corresponding to the envelope am- Thus, the distortioncontrol voltage appearing at the output of detector 52 varies as afunction of the absolute carrier frequency deviation and inversely asthe distortion expected in an average AM receiver.

A potentiometer tan is connected from the output terminal 52 to ground,with the variable tap being ected to the control electrode ofpredistortion control si nal plifier tube 191. Adjustment ofpotentiometer provides adjustment of the predistortion control signalamplitude, thereby providing effective control of the notch depth in theresponse characteristic of the notch filter network. The anode of tuberun is connected to a source of 8+ by a load resistor Hi2. and isfurther connected through a coupling capacitor 3% to the outnut terminal65 which corresponds to the terminal 65 of FIG. 4. Accordin ly thecircuit of FIG. 5 provides, at terminal 65, a predistortion controlsignal which varies as a function of the absolute carrier frequencydeviation and which corresponds to the similar signal applied to theterminal 65 in the system of FIG. 4.

From inspection of curve Bl in FIG. 6 and f in 1 1G. 7, it will beappreciated that the distortion precompensation systems of the presentinvention provide predistortion of the amplitude modulationsubstantially corresponding inversely to the distortion expected in aconventional am- Such precornpensation of the radiated signal enablesgreater carrier frequency de viation for a given level of cross talk ofthe PM signal into the AM reception.

The frequenc selective network of a conventional fi l l receiverexhibits approximately 8 kilocycles band-width between the 6 decibelattenuation points and as shown by curve Sit in 6. However, allamplitude modulation receivers do not have the same frequency responsecharacteristic. Some may have a narrower frequency responsecharacteristic such as exemplified by curve 37. Others may have anunusually wide frequency response characteristic as illustrated by curve35 which indicates bandwidth of approximately 12 kilocycles between the6 decibel attenuation points. Accordingly, it is not possible tocompletely and exactly precompensate the radiated carrier signal for thedistortion expected in all receivers. However, the followingcalculations show that a precornpensation system substan. y thefrequency response characteristic shown by the curve 41 in 7 willpartially compensate for the distortion expected in receivers havingeither wider or narrower than average bandwidths, whereby the crosstalkoccurring in such receivers is kept at reasonably tolerable levels.

An approximate formula for the relative gain of a receiver having afrequency response characteristic substantially corresponding to thesame characteristic of a high Q single tuned circuit is:

. 1 Relative gain=- where af is frequency departure from centerfrequency, and M is frequency departure of 3 db point from centerfrequency, or

where sf is frequency departure from center frequency, and A1 isfrequency departure of 6 db point from center frequency.

Since such an average amplitude modulation radio receiver has abandwidth of approximately 8 kilocycles at the 6 decibel attenuationpoints, M of Equation 2 will be equal to 4 kilocycles and the change inamplitude of a signal in response to a deviation of 3 kiiocycles will beapproximately:

l -2-:-: K/ GE) v2.69

Thus, the amplitude of a received signal will vary from 100% at centerfrequency to approximately 61% at 3 kilocycle peak deviation. Thatcorresponds to a spurious amplitude modulation resulting from thefrequency modulation of 24.2%.

That is;

A narrower than normal bandwidth receiver such as exemplified by curve37 in 6 and having a bandwidth of approximately 6 kilocycles between the6 db attenuation points may be analyzed by similar calculation. The 6kilocycle bandwidth will vary the amplitude of a frequency modulationsignal from:

or from a full amplitude of 1.0 at the center frequency t to 0.755amplitude at the 3 kilocycle deviation points. That spurious change inthe output amplitude of the carrier wave corresponds to an amplitudemodulation of 14%.

That is;

If the precompensation system is used in the transmitter having afrequency response characteristic substantially as shown by curve ll ofFIG. 7, the average amplitude modulation receiver exemplified by curve31 of FIG. 6 and Equations 3 and 4- will be perfectly compensated sincethe aoeasvo 13 predistortion control signal will boost the amplitudemodulation of the carrier as transmitted by a factor of at the times ofpeak deviation of the carrier wave from the center frequency f When theso boosted signal is received by a receiver having a narrower thanaverage bandwidth such as the 6 kilocycle bandwidth exemplified by curve37 of FIG. 6, the IF carrier wave at the receiver second detector Willcontain spurious amplitude modulation which is distorted by a factor of0.5, and is precorrected by a factor of Thus, the output variation willbe:

Thus, the output will vary from 0.82 to 1.0 and the actual distortionproduced by such a receiver responding to the precompensated signal willbe:

Thus, the actual distortion in such a receiver is only 9.9% as comparedto 33% resulting from an uncompensated frequency modulated signal.

A 12 kilocycle bandwidth AM receiver exemplified by curve 39 in FIG. 6Will be overcompensated and amplitude will vary from:

-lfil at maximum deviation, to 1.0, or an amplitude modulation of:

Thus, the compensation provided at the transmitter reduces thedistortion caused by such a receiver from 14% to 9%.

It will, of course, be appreciated that practically all the receivershave passbands closely corresponding to average bandwidth as shown bycurve 31, and that the curves 37 and 39 exemplify extreme examples whichoccasionally might be encountered. Consequently, the compensationprovided by the transmission of the system of the present invention willbe correspondingly better in most receivers. It will be noted that theprecompensation provided by the systems of the present invention notonly reduces distortion in the output of an ordinary amplitudemodulation receiver which is being used to listen monophonically, butalso enables the use of standard and economical circuits for thefrequency selective portion of a stereophonic receiver such as thatshown in FIG. 2.

The embodiments of the present invention as shown in F165. 4 and arebelieved to be particularly advantageous in that they may be constructedby addition of auxiliary components to cXisting broadcast band amplitudemodulation transmitters. More specifically, the expensive, high poweredclass C amplifier and amplitude modulator are already pro-existing inordinary amplitude modulation transmitters. Such a pre-existingtransmitter may be converted to the system of FIG. 4 simply by additionof the relatively inexpensive components necessary to frequency modulatethe carrier and to provide the predistortion control voltage formodifying the amplitude modulation.

The dual channel stereophonic receiver, illustrated herein at Fit}. 2 ofthe accompanying drawing, may correspond in circuit detail to thereceiver of concurrently filed application for US. patent, Serial No.898,037, filed April 22, 1959, entitled Broadcast Stereo Receiver, whichis assigned to the same assignee as that of the present invention. ingeneral, any techniques there described for reception of the signals astransmitted by the systems of the present invention may be utilized inthe system shown in FIG. 2 of the accompanying drawing.

While the present invention has been described with reference to variousspecific embodiments only, it will be obvious to those skilled in theart that it is not so limited but is susceptible of various changes andmodifications without departing from the spirit and scope thereof.

We claim as our invention:

1. A multiplex communication system comprising means to generate radiofrequency oscillation Waves, means to frequency modulate said waves inaccordance with one signal, means to amplitude modulate said waves inaccordance with another signal, the frequency deviation produced by saidfrequency modulation means being greater than the linear region of theamplitude-frequency response characteristic of an ordinary radioreceiver so that said response characteristic causes distortion of theamplitude modulation, and means coupled to said amplitude modulatingmeans for predistorting said amplitude modulation as a function of saidfrequency deviation and substantially inversely in accordance with thedistortion expected at the receiver, whereby the signal corresponding tothe amplitude modulation will be detectable in an ordinary receiverwithout substantial interference from the frequency modulation.

2. In a multiplex communications system a source of intelligence bearingfirst signal; a source of intelligence bearing second signal; meansresponsive to said first sig nal for generating a frequency modulatedcarrier having a predetermined center frequency and frequency deviationscontinuously substantially proportional to said first signal, with saidfrequency deviations being of such magnitude that the frequency responsecharacteristic of an ordinary broadcast band amplitude modulationreceiver generates an undesired amplitude modulation distortion inresponse to said carrier; amplitude modulation means to modulate theenvelope of said carrier in accordance with said second signal; andmeans including a notch filter network responsive to said frequencydeviations coupled to said amplitude modulation means to predis' tortsaid envelope in accordance with said frequency deviations andsubstantially inversely in proportion to said undesired amplitudemodulation distortion, whereby said second signal will be reproduciblein an ordinary amplitude modulation receiver without interference fromsaid frequency modulation and said first signal will be reproducible ina frequency discriminating receiver tuned to said predetermined centerfrequency.

1. in a communications system, a source of first modulation signal, asource of second modulation signal, means responsive to said firstmodulation signal for generating a frequency modulated carrier having apredetermined center frequency and frequency deviations continuouslysubstantially proportional to said first modulation signal; means toamplitude modulate said frequency modulated carrier in proportion tosaid second modulation signal; means responsive to said frequencymodulated carrier for generating a predistortion control signal whichvaries as a function of said frequency deviations; means for utilizingsaid predistortion control signal to further amplitude modulate saidcarrier whereby the amplitude modulation of said carrier is predistortedsubstantially inversely in accordance with the amplitude-frequencydistortion expected to occur in receiving said carrier in a conventionalamplitude modulation receiver, and means for transmitting said frequencyand amplitude modulated carrier.

4. In a multiplex communications system, a source of intelligencebearing first signal, a source of intelligence bearing second signal,means responsive to said first sigaccents ll .2 nal for generating afreque, cy modulated carrier having a predetermined center frequency andfrequency devia ions continuously substantially proportional to saidfirst signal, with said frequency deviations extending over a frequencyrange which exceeds the linear amplitudefrequency response range ofconventional broadcast band amplitude modulation receivers; meansresponsive to said carrier for generating a predistortion controlvoltage which varies continuously as a function of the carrier frequencydeviation from said center frequency; means responsive to said secondsignal for amplitude modulating said carrier substantially in accordancewith said second signal; and means responsive to said predistortioncontrol voltage for adding an amplitude modulation componentcorresponding inversely to the amplitude-frequency distortion expectedat said receivers, whereby said second signal will be reproduced inconventional amplitude modulation receivers without crosstalk from thefrequency modulation.

5. Apparatus for stcreophonic transmission of sound on a single carriercomprising first and second sound sources, means to add the outputs ofsaid sources to obtain a sum signal, means to subtract the said outputsto obtain a difference signal, means for generating a carrier having apredetermined center frequency, means for frequency modulating saidcarrier as a function of said difference signal, means responsive to thefrequency modulated carrier for producing a distortion precorrectionsignal which varies as a function of the frequency deviation of saidfrequency modulated carrier from said center frequency; transmitterapparatus for radiating said carrier including means for continuouslymodulating the radiated carrier energy; and means for concurrentlyapplying said sum signal and said correction signal to control saidcarrier energy modulating means so that the radiated carrier ener yvaries as a composite function of said sum signal and said precorrectionsignal and includes an amplitude modulation precorrection componentopposite and complernentary to the amplitude-frequency distortion whichwill occur in a conventional amplitude modulation receiver responding tosaid radiated carrier energy.

6. In a stereophonic radio system wherein first and second sound signalsare transmitted as separate modulations of a common carrie transmitterapparatus comprising means for generating a carrier having apredetermined center frequency, means to modulate the frequency of saidcarrier in accordance with said first signal, means including a notchalter network responsive to the frequency modulated carrier fordeveloping a control signal representative of the absolute differencebetween said center frequency and the instantaneous frequency of saidmodulated carrier, means to provide amplitude modulation of saidfrequency modulated wave in accordance with said second signal, means tomodify said amplitude modulation in accordance with said control signal,and an antenna system for radiating the amplitude and frequencymodulated carrier wave; radio receiver means comprising first and secondchannels having an amplitude detector and a frequency detectorrespectively, said amplitude detector yielding said second signal andsaid frequency detector said first signal, said receiver means furtherin cluding a frequency selective network with said amplitude detectorbeing coupled to the output of said selective network, said selectivenetwork having a non-linear frequency response characteristic over therange of frequency deviations of said modulated carrier, said notchfilter network having a non-linear transfer characteristic substantiallycorresponding inversely to the characteristic of said selective networkover said range of frequency deviations, and means coupled to saiddetectors for separately reproducing said first and second soundsignals.

7. in a multiplex communications system, a source of intelligencebearing first signal, a source of intelligence bearing second signal,means responsive to said first signal for generating a frequencymodulated carrier having a predetermined center frequency and frequencydeviation continuously varying substantially as a function of said firstsignal, amplitude modulation means to modulate the envelope of saidcarrier in accordance with said second signal, means including a notchfilter network responsive to said frequency deviation coupled to saidamplitude modulation means to predistort said envelope in accordancewith said frequency deviations, and means for transmitting saidfrequency and amplitude modulated carrier; radio receiving meansresponsive to said carrier at a remote location, said receiving meanscomprising a frequency selective network and first and second channelsand respectively including amplitude demodulation means and frequencydemodulation means for responding to carrier signals translated by saidnetwork to reproduce said first and second intelligence bearing signalsrespectively, said frequency selective network having anamplitudefrequency response characteristic which is non-linear over therange of said frequency deviations, and said notch filter network havinga non-linear transfer characteristic substantially correspondinginversely to said response characteristic over said range of frequencydeviations, whereby the amplitude modulation predistortion introdt cedby said notch filter network substantially counteracts the amplitudedistortion introduced by said frequency selective network so illci saidamplitude demodulation means reproduces said second signal withoutsubstantial crosstalk interference from said frequency modulation.

8. In a multiplex communication system wherein first and second signalsare transmitted and received respectively as frequency and amplitudemodulations of a common carrier; transmitting apparatus comprising asource of sound representative signal A, a source of soundrepresentative signal B, difference producing means for combining thesignals A and B to produce a difference signal A-B, sum producing meansfor combining the signals A and B to provide a sum signal A-l-B, meansresponsive to said difference signal A-B for generating a frequencymodulated carrier having a predetermined center frequency and frequencydeviations continuously varying substantially as a function of saiddifference signal, means including a notch filter network responsive tosaid frequency modulated carrier for producing a carrier wave having anamplitude modulation envelope which varies as a function of the absolutedifference between said center frequency and the instantaneous frequencyof said frequency modulated carrier, amplitude demodulation meanscoupled to said notch filter network responsive to said carrier wave toproduce a distortion preco-rrection signal which varies as a function ofthe frequency deviation of said frequency modulated carrier from saidcenter frequency, circuit means coupled to said sum producing and tosaid demodulation means for additively combining said sum signal A-l-Bwith said distortion precorrection signal to produce a predistortedmodulation signal, amplitude modulation means coupled to said carriergenerating means and responsive to said predistorted modulation signalfor modulating the envelope of said carrier, and means for transmittingsaid frequency and amplitude modulated carrier, remotely located radioreceiving means responsive to said transmitted carrier comprising afrequency selective network having an amplitude-frequency responsecharacteristic which is non-linear over the range of frequencydeviations of said carrier, first and second signal channelsrespectively including amplitude demodulation means and frequencydemodulation means respectively responsive to carrier signals translatedby said frequency selective network toreproduce respectively said sumsignal and said difference signal, said notch filter network having anonl. ear transfer characteristic substantially corresponding inverselyto said receiver response characteristic over said range of carrierfrequency deviation, whereby the amplitude modulation predistortionintroduced by said distortion precorrection signal substantiallycounteracts the amplitude distortion introduced by said receiverresponse characteristic so that said amplitude demodulation meansreproduces said sum signal Without substantial frequency intermodnlationdistortion, a sum producer and a differ ence producer coupled in commonto said first and second signal channels and each responsive to said sumsig nal A+B and said dilierence signal A-B to recover respectively saidsound representative signals A and B.

9. In a multiplex communicaitons system, a source of intelligencebearing first signal, a source of intelligence bearing second signal,means responsive to said first signal for generating a frequencymodulated carrier having a predetermined center frequency and frequencydeviation continuously varying substantially as a function of said firstsignal, amplitude modulation means to modulate the envelope of saidcarrier in accordance with said second signal, means including a notchfilter network responsive to said frequency deviation coupled to saidamplitude modulation means to predistort said envelope in accordancewith said frequency deviations, and means for transmitting saidfrequency and amplitude modulated carrier; signal receiving meansresponsive to said carrier comprising a frequency selective network,amplitude demodulation means responsive to carrier signals translated bysaid network for reproducing said second signal, said frequencyselective network having a frequency response characteristic which isnonlinear over the range of said -requency deviations and said notchfilter network having a nonlinear transfer characteristic correspondinginversely to said response characteristic.

10. In combination in a system for transmitting first and second signalsas separate modulations of a single t8 carrier; means for generating acarrier having a predetermined center frequency, means for modulatingthe frequency of said car ler in accordance with said first signal,filter means having a nonlinear frequency response characteristic fordeveloping a control signal which varies as a predetermined function ofthe difference between said center frequency and the instantaneousfrequency of the frequency modulated carrier, means for amplitudemodulating said carrier in accordance with said second signal, means formodifying said amplitude modulation of said carrier in accordance withsaid control signal and means for transmitting the amplitude andfrequency modulated carrier; signal receiving means responsive to saidmodulated or for reproducing at least said second signal, said receivingmeans comprising a frequency selective network having a frequencyresponse characteristic Which is nonlinear over the range of frequencydeviations of said modulated carrier, with the frequency responsecharacter- -stic of said selective network being inversely related tolie response characteristic of said filter means.

References tilted in the tile of this patent UNITED S'EAI'ES PATENTSTransmission on a Single Channel, Eastman et a1.

